1. Field of the Invention
A developing device for developing latent images into toner images has been used in an image forming apparatus.
2. Description of the Prior Art
Conventional developing devices include one-component developing devices or two-component developing devices. One-component developing devices develop latent images to toner images with developer containing toner particles but not containing carrier particles. Two-component developing devices develop latent images to toner images with developer containing toner particles and carrier particles.
Two-component developing devices are suitable for a high-speed image forming apparatus and are broadly used in medium-speed and high-speed image forming apparatuses. It is common to pack the two-component developer with high density when the developer touches latent images, in order to form a high quality image. To pack the two-component developer with high density, the diameter of carrier particles is reduced in size. Carrier particles with the diameter of 30 μm are commonly used. However, demand for high quality image has grown stronger, with the size of one pixel required to be equal to or smaller than the size of conventional carrier particles. Therefore, in terms of the reproducibility of isolated dots, it is desirable to reduce the size of carrier particles further.
However, when carrier particles are downsized, the magnetic permeability of carrier particles tends to decline, resulting in carrier particles dropping from a development roller during image production. If a carrier particle dropped from the development roller attaches to a latent image carrier, it will result in image distortion caused by adhesion. Dropped carrier particles also will result in additional side effects such as injury of the photoconductor. Various efforts has been tried to reduce the adhesion of dropped carrier particles. For example, research has been performed relative to material having higher magnetic permeability. Research has also been conducted to raise the magnetic power of magnets disposed within the development roller. However, it is difficult to find a solution that meets a requirement for low cost and a requirement for high quality of image at the same time. Moreover, with the trend of downsizing the developing device, the development roller is getting smaller and it is even more difficult to design a development roller having a strong enough magnetic field to suppress carrier particles.
In addition, two-component developing devices develop toner images with developer particles that are arranged along a magnetic field, thus forming a chain-like shape called a “magnetic brush” so that the developer particles are in contact with and rub latent images. This development process tends to result in the distorted toner image because of the unevenness of conventional magnetic brush density. Although it is possible to improve the image quality by applying an alternating bias between the development roller and the latent image carrier, it is difficult to remove the distortion of toner images caused by the unevenness of the magnetic brush in density.
In addition, it is desirable to reduce non-electrostatic adherence between toner particles and the latent image carrier when toner particles are transferred or removed from the latent image carrier. It is well known that the non-electrostatic adherence between toner particles and the latent image carrier can be reduced effectively by reducing a friction coefficient of the surface of the latent image carrier. However, reducing a friction coefficient tends to reduce the efficiency of development or reproducibility of isolated dots because the magnetic brush passes through the latent image carrier too smoothly.
On the other hand, one-component developing devices are frequently used in conventional low-speed image forming apparatuses because it has advantages of mechanical simplicity and easy downsizing. In conventional one-component developing devices, a regulating blade or roller is pressed to a development roller to form a thin layer of toner particles on a development roller, and toner particles are electrically charged due to the friction with the regulating blade, roller or the development roller. The electrically charged thin layer of toner particles is carried to a development area in which the development roller faces the latent image carrier.
The conventional one-component developing device is classified as a contact type or a non-contact type. In a contact type one-component developing device, the development roller is contact with the latent image carrier. In a non-contact type one-component developing device, the development roller is not contact with the latent image carrier. Toner particles are compressed on the development roller in either type of developing device, making it difficult for the toner particles to move smoothly in response to an applied electric bias. Therefore, a strong alternating bias is often applied between the development roller and the latent image carrier in order to obtain the high quality image. However, even when a strong alternating bias is applied, it is still difficult to stabilize the amount of toner particles to be adhered on to a certain area of latent image and to develop an isolated dot with high resolution uniformly. In addition, toner particles tend to deteriorate easily because of a strong stress that toner particles suffer when the thin layer of toner particles are formed. Once toner particles deteriorate, the thin layer of toner particles tends to be uneven. Therefore, one-component developing devices are not suitable for a high-speed image forming apparatus or for long time use.
To address one or more of the issues described above, Japanese Laid-Open Patent Publication No. 3-100575 discloses a hybrid type of developing device in which a one-component developing device and a two-component developing device compensate for demerits of each other. Although this type of developing device requires a larger space and more components, it solves some problems explained above. However, this device has the same problem as the conventional one-component developing device in that it is difficult to develop an isolated dot with high resolution uniformly.
Japanese Laid-Open Patent Publication No. 3-113474 discloses a developing device with high frequency alternating bias on a wire. The alternating bias is applied to the wire causing toner particles to form a toner cloud and makes it possible to improve the reproducibility of an isolated dot with high resolution. Although this type of the developing device requires a complicated structure, it makes it possible to develop latent images with high image quality stably.
Japanese Laid-Open Patent Publication No. 3-21967 discloses a developing device configured to form a toner cloud stably and effectively by forming an electric field curtain on a rotating roller. This type of developing device is suitable for obtaining toner images with high image quality and suitable for downsizing at the same time. However, this device requires specific set up conditions for forming an electric field curtains in order to obtain high image quality. If the development process is executed under wrong set up conditions, the image quality can be worse than that of other type of developing device.
Japanese Laid-Open Patent Publication No. 2002-341656 discloses another developing device configured to form a toner cloud. This developing device transports toner particles electrically with a driving alternating electric field of 3 or more phases instead of conveying toner particles mechanically by a toner carrier.
In order to stabilize the amount of electrical charge of toner particles, Japanese Laid-Open Patent Publication No. 2004-205644 proposes to provide a storage area in which toner particles are electrically charged by electrical voltage before being transported to an electrostatic transport substrate.
However, the developing devices shown in Japanese Laid-Open Patent Publication No. 2002-341656 and Japanese Laid-Open Patent Publication No. 2004-205644 each suffer certain deficiencies. Toner particles with weak charge tend to stay on the electrostatic transport substrate because toner particles with weak charge do not respond to electric field well. Therefore, a cleaner is required to remove toner particles on the electrostatic transport substrate.
In addition, the amount of toner particles transported on the electrostatic transport substrate tends to fluctuate in response to fluctuation of the charge amount of toner particles. The developing device shown in Japanese Laid-Open Patent Publication No. 2004-205644 is useful to enlarge the charge amount of toner particles but is not useful to make the charge amount of toner particles even. Therefore, for keeping high image quality, there is also a need to make the charge amount of toner particles even.